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Thread: Solar Hot Water Install

  1. #1
    House SOB Little Corn Tom's Avatar
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    Default Solar Hot Water Install

    These 2 solar panels are what I recently installed on my roof.

    So far, they can generate as much as 700 watts which I have wired directly to my hot water heater. Hot water heaters have 2 heating elements and I am using one element for solar power. The other element is turned off or on at the circuit breaker panel in case of extended bad weather or holocaust.

    No battery and no controller. This is strictly dedicated to hot water heating. We have not used anything but solar hot water in a month so far and this is the rainy (cloudy) season.

    Total cost of parts has been around $600. Estimated savings is $130 per year (3 KWH / Day @$.12 per KWH).

    Pretty simple install with no batteries and controller complications. Did not pull permits or seek permissions of any kind of course.

    The hot water heater upper heating element (there are two heating elements) has been switched off for almost a month and I don't fiddle with it at all, ever. The solar panels are hooked to the lower element. We have a small , 40 gallon tank and it seems to lose 5-10 degrees over night.

    Morning tank temperatures have been as low as 95 and high as 116. Evening showers can be real damn hot so it requires turning on cold water first and adding hot water to suit.

    We mostly wash clothes in cold water, dishes in hot water, showers in hot water but Maria seems to be quite frugal there.....I am not. Only 2 of us, so a 40 gallon tank is fine.

    Keep in mind that 30 days does not a year make, so my answers may change.

    The solar panel mounting brackets are variable and I anticipate adjusting these 2 times a year. You can see from the shadows in the picture that the angle is about optimum now for this time of year.

    Frankly, I am surprised at the simplicity of the installation and its effectiveness, as is. Of course, time will tell.

    One thing they don't tell you is that you must "tune" your heating element to the solar panels. After you decide on the voltage and current in your configuration (different panels have different output voltages and output current) then you must pick a heating element to match. It does make a difference in performance.

    I opted to configure this with 2 solar panels in series or about 80 volts and 10 amps. Keep in mind that this is DC. Turns out regular a/c switches cannot handle dc switching, so I found an Airpax circuit breaker / switch that will handle the load and switch also....works great.

    Sorry about the sideways picture....don't know how to flip it on this site.P1010138.jpg
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    Last edited by Little Corn Tom; 08-24-2020 at 02:39 PM.
    Life's different here ... It's a whole 'nother pace.

  2. #2

    Default Re: Solar Hot Water Install

    "Turns out regular a/c switches cannot handle dc switching,"

    Nor can the thermostats on the hot water heater handle DC ,, which I discovered to my dismay. I need to buy a replacement,, had forgot it. KISS is always best. Whatever gets the job done.

  3. #3
    House SOB Little Corn Tom's Avatar
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    Default Re: Solar Hot Water Install

    Oh crap.....I forgot about the thermostat.....excuse me but I have to go change a wire....lol....thanks.
    Life's different here ... It's a whole 'nother pace.

  4. #4

    Default Re: Solar Hot Water Install

    Turn the thermostat up to 140.
    That is the battery you don't think that you have,,

    >>> Just don't the into the shower before you adjust water temp


    I used to run the dishwasher right after lunch,, about noon,, after dinner the night before, breakfast, and lunch dishes.

    It was a great system, but Lupe could never figure it out.

    For the entirety of our life together, either Shelley or I would run the dishwasher right before we went to bed. It was part of the lock the door ritual, but it can mean a cold shower in the morning if your power comes from solar panels.

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    House SOB Little Corn Tom's Avatar
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    Default Re: Solar Hot Water Install

    Yeah, I had already turned the thermostat way up.
    Life's different here ... It's a whole 'nother pace.

  6. #6
    TRN Science officer bill_bly_ca's Avatar
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    Default Re: Solar Hot Water Install

    Quote Originally Posted by KeyWestPirate View Post
    "Turns out regular a/c switches cannot handle dc switching,"

    Nor can the thermostats on the hot water heater handle DC ,, which I discovered to my dismay. I need to buy a replacement,, had forgot it. KISS is always best. Whatever gets the job done.
    Well they can, just not at the rated AC power - I want to say it is 0.707 of the AC value for DC working - But that theory I took was 35 yrs ago so I have to look it up.

    So your AC 15A breaker is really 10A DC, your 1500W AC element you can only supply 1000W DC etc etc..

    Have to google that to be sure.
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    TRN Science officer bill_bly_ca's Avatar
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    Default Re: Solar Hot Water Install

    Oh yea - Nice set up as well.
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    TRN Science officer bill_bly_ca's Avatar
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    Default Re: Solar Hot Water Install

    You also might want to throw a big A$$ DPDT switch on there as Galvonic corrosion may occur an a AC element run DC - Just flop over polarity every month or so. Just a guess on my part , but I guess less so if you ran PEX and the tank is electrically isolated..
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    House SOB Little Corn Tom's Avatar
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    Default Re: Solar Hot Water Install

    Quote Originally Posted by bill_bly_ca View Post
    Well they can, just not at the rated AC power - I want to say it is 0.707 of the AC value for DC working - But that theory I took was 35 yrs ago so I have to look it up.

    So your AC 15A breaker is really 10A DC, your 1500W AC element you can only supply 1000W DC etc etc..

    Have to google that to be sure.

    Nope...the issue is the fact that 60 times a second, ac voltage and current is zero and it is easy to interrupt current flow and the damaging arcing, But with dc, it will just keep arcing so dc switches need more spacing, bigger contacts, and stronger springs.

    I gotta think about the galvanic issues....I never really understood them very well and no mention of them in any posts.
    Life's different here ... It's a whole 'nother pace.

  10. #10
    TRN Science officer bill_bly_ca's Avatar
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    Default Re: Solar Hot Water Install

    Nothing mentions Galvanic corrosion and at 80V you have no worries of toasting the element. Good show..
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  11. #11
    TRN Science officer bill_bly_ca's Avatar
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    Default Re: Solar Hot Water Install

    Quote Originally Posted by Little Corn Tom View Post
    Nope...the issue is the fact that 60 times a second, ac voltage and current is zero and it is easy to interrupt current flow and the damaging arcing, But with dc, it will just keep arcing so dc switches need more spacing, bigger contacts, and stronger springs.

    I gotta think about the galvanic issues....I never really understood them very well and no mention of them in any posts.

    Ah and so it is...
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  12. #12
    Para aquí para acá Jonh's Avatar
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    Default Re: Solar Hot Water Install

    Lawyer note: Nothing on TRN should be taken as literal advice. Don't eat Tide Pods. Consult local codes before attempting. Always obtain a building permit. Consult your doctor if hot water lasts more than four hours. May cause drowsiness.

  13. #13

    Default Re: Solar Hot Water Install

    Quote Originally Posted by bill_bly_ca View Post
    Nothing mentions Galvanic corrosion and at 80V you have no worries of toasting the element. Good show..


    Unless that is a 12 V element you put in there. Even then,, with that enormous heat sink it's sitting in,, probably can't burn it out.

    I accept that the load resistance should match the input voltage. I bought a 48 V element for my operation.
    However, I still have trouble wrapping my head around the concept. I can't do the math.


    OK,, this is a spec sheet from my favorite solar panel seller,, who happens to be just down the road from me> He has incredible deals from time to time,, closeouts, leftovers, labels removed,, non UL so can't be used Grid Tie. Whatever.

    https://www.ebay.com/itm/New-Seraphi...QAAOSw7NlfP-H7





    Rated Power: 370W
    Open circuit voltage (VOC): 48.0 V
    Max power voltage (VMP): 39.1 V
    Short circuit current (ISC): 9.96 A
    Max power current: 9.60
    Maximum system voltage: 1000V or 1500V UL




    OK,, so we know that a 220V element that comes with the standard water heater is low resistance. But the spec sheet claims a short circuit current of 9.96 amp. It's all about the watts realized, so I think that this is what we are looking for.




    I have 4.2 KW of 60 cell panels, strung together in threes. The individual panel VOC as I remember is 37,, which you will only see with a meter. Any load reduces that voltage. When the batteries are completely charged, and there is minimal load on the system,, with bright sun, early afternoon,, I see 95 + volts input to my mppt controller.

    During normal operation, I most frequently see 85 volts when the panels are really cranking out the power. i get that maybe 3 hours,, from 11 AM to 2 PM. On a good day.

    But,, by stringing the panels in series,, I get something as early as 7AM. It's not much, but at that time of day,, everything helps.


    I'd like to buy more panels,, but the killer deals now are all 72 cell units. Everything is Grid Tie in the US, or almost everything. The higher voltage needed for a grid tie drives the market. As those poor people in California who invested a huge sum to become electrically independent are finding out,,,, without the grid your system doesn't work --at all. No matter how much sun you have.

    Sure,, you can invest in a Tesla power wall and another -really big- inverter. So far, the outages have been minimal,, two hours, four hours. Who knows what will happen next month, when fire season officially starts ? We're only seeing a preview right now.
    A far better investment would be a large, natural gas generator. Cover your bases.

    The higher voltage is not a problem except the common mppt controllers have a max input voltage from the panels,, mine is 127. Higher voltage everything is now available,, but it wasn't when I embarked on this project ten years ago.

    So, I took out the thermostat (temporarily, the contacts froze together anyway). I have an elaborate switching setup using two forklift contactors, and a solid state relay slaved to my controller. It has an output that goes high when it senses the battery completely charged, and allows me to set how long the relay stays closed.

    It works OK, but at the moment, without a thermostat, I have to disconnect it when I'm not there. We use a propane on demand water heater to heat water. I have to buy propane, but only Krisnia is there at the moment. If I'm there I notice how hot the water is on a continuous basis, so the missing thermostat is not a problem.
    But,, If I'm NOT there, well,, there is that potential for disaster.

    It's small,, but Murphy's law ALWAYS applies.
    The water heater does have a pressure blowoff valve after all


    I'm thinking of just putting the water heater on it's own solar panel string. Four or five of the panels above strung in series would give me the 200 V for the two elements (which I still have, and are commonly available for purchase,, SINSA has them).

    If I slave the new thermostat to the fork lift contactors,, I SHOULD have a fool proof system.

  14. #14
    House SOB Little Corn Tom's Avatar
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    Default Re: Solar Hot Water Install

    For the above stated panels and using them in series gives you 2 x Vmp / Imp or 2 x 39.1 / 9.6 = 8.1 ohms. Your direct connected element should have a resistance of 8.1 ohms. This is assuming a direct connect with no controllers and no batteries.

    Zoro offers a wide variety of heating elements and their Model # SG2167L is 8.7 ohms.

    This would give you maximum power transfer for the most hot water and least complexity with minimal wire cost.

    https://www.zoro.com/zoro-select-wat...37/i/G1741853/
    Life's different here ... It's a whole 'nother pace.

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    TRN Science officer bill_bly_ca's Avatar
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    Default Re: Solar Hot Water Install

    Quote Originally Posted by KeyWestPirate View Post
    Unless that is a 12 V element you put in there. Even then,, with that enormous heat sink it's sitting in,, probably can't burn it out.
    I was thinking of electron flow, through mineralised water, to earth - But it looks like elements are coated enough so that does not occur

    Supplying 800W to a (most likely) 1500w element is going to give it a long healthy life..
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  16. #16

    Default Re: Solar Hot Water Install

    "For the above stated panels and using them in series gives you 2 x Vmp / Imp or 2 x 39.1 / 9.6 = 8.1 ohms. "


    Shit,, I'm going to have that tattooed someplace where I can refer to it.
    Beyond E=IR I really don't have much formal education.
    Now I know where to go for answers



    "I was thinking of electron flow, through mineralised water, to earth - But it looks like elements are coated enough so that does not occur "

    Most private DYI solar installations are not grounded. Why ? I suspect that it is a UL or CSA requirement for grid tie. but my thinking (and from some other blogs) is that it's just a lightning attraction. What does it buy you?

    Unfortunately, my zinc roof and steel construction with support posts embedded into the ground means that at least the aluminum frames are grounded.

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